With the advancement of technology, and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premise (CP) has become more and more important.
In a digital subscriber line (DSL) communication system, and more particularly an xDSL system, where “x” indicates a plurality of various standards used in the data transfer, data is transmitted from a CO to a CP via a transmission line, such as a two-wire twisted pair, and is transmitted from the CP to the CO as well, either simultaneously or in different communication sessions. The same transmission line might be utilized for data transfer by both sites or the transmission to and from the CO might occur on two separate lines.
A hybrid circuit is introduced at both the CO and the CP to separate the respective transmit signals from the CO and the CP from the receive signals at each end of the two-wire transmission line for the case where both the CO and the CP communicate with each at the same time over the two-wire transmission line. For example, a hybrid circuit at the CO serves as an electrical bridge that removes all but a small portion of the downstream data transmission intended for the respective CP from a receive signal transmitted in the upstream direction by customer devices (via a CP hybrid) at the CP.
Generally, in systems for transmitting data over twisted-pair loops, the hybrid circuit accomplishes this duplex filtering task with an appropriately configured electrical bridge. The electrical bridge or balance network is selected to match the two-wire twisted pair loop impedance. These two impedances, namely the twisted-pair loop impedance and the impedance of the balance network, should closely match in order for the hybrid to successfully prevent transmit signals from feeding into the receive signal path. Twisted-pair loop impedances are determined by wire type (i.e., gauge and material composition), loop length, and bridged taps. Bridged taps are sections of wire coupled to the twisted-pair two-wire loop not on the direct path between the CP and the CO.
Prior art hybrids with fixed passive balance networks have been optimized for installation with typical twisted-pair loops. However, these fixed balance network hybrids suffer from the disadvantage that twisted-pair loop impedances often stray dramatically from “typical.” Stated another way, fixed balance networks do not offer the flexibility required to match the impedance of the various twisted-pair loops encountered in the public switched telephone network (PSTN) and other various networks that communicate via a two-wire pair. Not only are fixed balance network hybrids disadvantageous when installed in association with a twisted-pair loop having one or more bridged taps, a different length or wire gauge, but changes in the loop impedance during operation result in less echo rejection of the local transmit signal from the receive path.
It will be appreciated, from the aforementioned disadvantages that result from the inflexibility of a fixed balance network, that an adaptable balance network is desirable. An adaptive balance network is described in an article by Pecourt et al., entitled, “An Integrated Adaptive Analog Balancing Hybrid,” IEEE Solid State Circuits Conference, San Francisco, 1999. The adaptable balance network disclosed by Pecourt et al. discloses an implementation that adaptively adjusts the entire balance network of the hybrid.
The Pecourt et al. solution of adaptively adjusting the entire balance network consumes significant computing resources. Furthermore, Pecourt's methodology fails to take advantage of telephone industry standards that dictate the electrical properties of the line transformer and the tip and ring circuits.
The Pecourt et al. solution makes several assumptions, which lead to a problematic circuit that does not perform as Pecourt indicated when applied in an ADSL system. ADSL service co-exists on a transmission line with POTS (or ISDN). As a result, the input to an ADSL modem or transceiver must have a frequency dependent filter (in this case a high-pass filter) so as to not disturb the POTS frequency band. (Frequently an external splitter is added that effectively performs the filtering.) The impedance of the high-pass filter or external splitter in combination with the transmission line is mandated by International Telecommunication Union (ITU) standards. The required frequency dependence means that the hybrid network must match not just the transmission line, but the transmission line as viewed through the isolation transformer and the high-pass (or external splitter).
Furthermore, ADSL is usually operated in a frequency-division multiplex (FDM) mode, (i.e., ADSL separates up-stream and down-stream frequency bands). As a result, the up-stream and down-stream data rates are limited by noise rather than transmission echo (as is the case for symmetric DSL services, which use the same frequency band for up-stream and down-stream signal transmissions), hence the noise level of any adaptive hybrid is crucial.
Pecourt et al. assumes that the line impedance can be matched with a 1St order filter and that a 2nd order filter can be used in the presence of a bridged tap. Furthermore, Pecourt et al. indicates that for the ADSL customer side, the frequency range of interest is limited to 150 kHz. These two assumptions are incorrect: the high-pass filter impedance must be taken into account, and matching above 150 kHz cannot be ignored.
The isolation transformer and high-pass filter increase the order of the matching function, so that the matching function behaves as a 3rd order function even in the absence of bridged taps. A bridged tap or other impedance effect on the transmission line increases the order of the matching function beyond a 3rd order function.
The hybrid cannot “stop working” at 150 kHz (the upper limit of the CP transmitter band for ADSL over POTS is 138 kHz) because the transmitter will transmit noise and distortion throughout the receive band (138 kHz to 1104 kHz for ADSL with POTS). This interference will corrupt the receive signal unless it is suppressed by the hybrid. (This fact is mentioned in the Pecourt article.) It is true that the AGC gain is set mainly by the echo, but if the frequencies outside of the echo are drowned in transmit signal related noise, remotely generated signal transmissions will be difficult to recover.
The Pecourt et al. article describes a system where the full hybrid is implemented on an integrated circuit or chip. This implementation requires prohibitively large capacitors in the circuit (C1 and C2 in Pecourt's FIG. 14.8.4) to achieve good noise levels. Furthermore, when the requisite high-pass filter and isolation transformer are added, there are no degrees of freedom “left” for the bridged taps, etc. In fact, Pecourt's solution does not offer sufficient flexibility to match the combination of the isolation transformer and high-pass filter. Moreover, Pecourt's solution does not function above 150 kHz, where the remotely generated or receive signal gets corrupted by the local transmit signal noise and distortion.